In order to obtain increased corrosion protection as well as other advantages (e.g. better sacrificial protection of steel; improved formability, weldability and paintability) efforts have been undertaken in the field of zinc coatings to develop improved zinc alloys for the continuous or batch application to substrates. Studies carried out in this direction have resulted in the development of new types of coatings such as the alloy Zn-55 Al-1.5Si and other zinc alloys having low (i.e., less than 15%) Al-1.5Si content. The Zn-55 Al alloy coating developed by Bethlehem Steel (see for example U.S. Pat. Nos. 3,343,930 and 3,393,089) reportedly exhibits a good corrosion resistance but, in view of its high aluminum content does not provide a satisfactory sacrificial protection of the steel substrate.
Subsequent studies have been aimed at modifying the composition of molten metal baths in order to form (by hot-dipping) a coating which improves corrosion resistance even in the most varied environments. One of the aspects of these studies was the influence of the preparation of the surface to be coated on the quality of the product obtained. It thus appears that in order to ensure a quality coating, some of the alloy coatings previously developed required expensive preliminary surface treatments involving expensive equipment. For example, this was the case with respect to zinc coatings containing typically about 5% Al and additions of other elements such as Sb, Pb+Mg, and Pb+Mg+Cu proposed by Inland Steel (see for example Inland U.S. Pat. Nos. 4,029,478 and 4,056,366 as well as U.S. Pat. No. 4,152,472 assigned to Nippon Steel). There exists evidence showing that compositions of these types are characterized by a pronounced tendency to form bare-spots and similar defects even in the presence of careful surface preparation.
In view of the above considerations, there continues a need for a hot-dip metal bath of such composition that no special or expensive surface preparation of the substrate would be necessary and such that the protective coating obtained thereby is substantially free of bare spots or other defects.